1. Field of the Invention
The present invention relates generally to high-speed source-transfer switching systems and more particularly to a control arrangement and method that controls the transfer of a load from one source to another in a desirable fashion to minimize transfer delays while also avoiding undesirable current flow between the sources.
2. Description of Related Art
High-speed source-transfer switching systems (HSSTSS) for electrical power distribution systems provide reliable, continuous power delivery to a load by transferring the supply of the load from a first source to a second independent source when undesirable characteristics are sensed in the first source. To achieve high-speed transfer operation, one type of HSSTSS utilizes solid-state switches formed by thyristors, one solid-state switch for each of the sources. These high-speed switches are also known as static transfer switches. To control the transfer operations, the HSSTSS utilizes control arrangements to provide appropriate control signals to control the operation of the thyristors of each solid-state switch via the gate of each thyristor. The control arrangements sample the voltage waveforms of each source to detect when transfer between the sources is necessary, e.g. sensing outages and momentary interruptions as well as voltage sags and swells based on the source supplying the load being above or below preset levels. Under certain circuit conditions, transfer between sources by the control arrangements can introduce undesirable transfer delays and/or permit undesirable current flow between the sources. The arrangement in U.S. Pat. No. 5,808,378 avoids undesirable current flow between sources by delaying transfer until a polarity comparison is satisfied that ensures that the current will flow in the incoming source after transfer will be in opposition to the current flowing in the outgoing source before transfer. Undesirable transfer delays are also minimized by establishing a forced commutation condition before transfer between sources, i.e. transfer is delayed until the incoming source voltage differential and the outgoing current establish an initial change in current flow in opposition to current flow in the switch to be turned off.